Parts, particularly parts including complex geometry, may be fabricated using an investment casting process. In a typical investment casting process, a master pattern is fabricated to correspond in shape (but not necessarily in dimensions) to a desired finished part, and the master pattern may be made of a durable material, such as aluminum. The master pattern may be stored in a facility that may be remote from or at the facility where the finished part is fabricated. When a part (or an assembly including the part) is ordered by a customer, a corresponding master pattern is located and used to build a mold (known as a master die) having a cavity that corresponds in shape to the master pattern. The master die may be made using any of several processes and materials, such as creating a silicone mold of the master pattern. Molten wax is poured into the cavity of the master die to make a wax pattern that corresponds in shape to the master pattern. The wax pattern is then treated to erase parting lines, flashing, and/or any other imperfections. A wax sprue—which defines a pathway for a molding material to enter a cavity of an investment mold—may then be coupled to the wax pattern.
The investment mold (also known as a shell) is next fabricated in a process called shelling, and the shell is typically made from a ceramic material and produced by repeating a series of steps—coating, stuccoing, and hardening—until a desired mold thickness is achieved. Coating involves dipping the wax pattern (and the sprue) into slurry of fine material to create a uniform surface coating. In the stuccoing process, coarse ceramic particles are applied over the fine material. After a desirable thickness has been achieved, the coatings are then allowed to cure in the hardening stage.
Once the shell has fully cured, it is turned upside-down and placed in a furnace or autoclave to melt out and/or vaporize the wax pattern. Most shell failures occur at this point because the wax used often has a thermal expansion coefficient that is much greater than the shell material surrounding it. Accordingly, as the wax is heated, it expands and introduces stresses to the shell. To minimize these stresses, the wax is heated as rapidly as possible so that outer wax surfaces can melt and drain quickly, making space for the rest of the wax to expand.
The shell may then be pre-heated and a molding material—typically a molten metal—may be poured into the cavity of the shell left by the melting of the sprue and wax pattern. The molten metal may be gravity poured or forced by applying positive air pressure or other forces into the sprue (and/or any other inlet) of the shell. The shell may then be placed into a tub filled with sand to cool. When the molten metal has hardened and reached a suitable temperature, the shell is then destroyed to reveal the finished part. The finished part may then be cleaned up to remove signs of the casting process (e.g., remove the sprue and/or grind out surface imperfections) and shipped to a further location for assembly or storage.
The finished part may be fabricated using any of several materials. Because each material may have a different coefficient of thermal expansion, a unique master pattern—each having a unique set of dimensions—may be required to correspond to each material used to fabricate the same metal part. For example, if a first metal material (having a first coefficient of thermal expansion) is used to fabricate a finished part having specified dimensions, a unique first master pattern must be used to create the corresponding wax pattern. However, if a second metal material (having a second coefficient of thermal expansion) is used to fabricate a finished part having the same specified dimensions as the finished part made from the first metal material, a unique second master pattern must be used to create that corresponding wax pattern. Accordingly, for each finished part that can be created out of different materials, two or more master patterns must be fabricated and stored. This system results in relatively high manufacturing costs and complicates the manufacturing process. In addition, creating the master wax mold adds significant time to the production of a finished part.